1. Field of the Invention
This invention relates to an oil pump rotor assembly used in an oil pump which draws and discharges fluid by volume change of cells formed between an inner rotor and an outer rotor.
2. Background Art
A conventional oil pump comprises an inner rotor having “n” external teeth (hereinafter “n” indicates a natural number), an outer rotor having “n+1” internal teeth which are engageable with the external teeth, and a casing in which a suction port for drawing fluid and a discharge port for discharging fluid are formed, and fluid is drawn and is discharged by rotation of the inner rotor which produces changes in the volumes of cells formed between the inner rotor and the outer rotor.
Each of the cells is delimited at a front portion and at a rear portion as viewed in the direction of rotation by contact regions between the external teeth of the inner rotor and the internal teeth of the outer rotor, and is also delimited at either side portions by the casing, so that an independent fluid conveying chamber is formed. Each of the cells draws fluid as the volume thereof increases when the cell moves over the suction port after the volume thereof is minimized in the engagement process between the external teeth and the internal teeth, and the cell discharges fluid as the volume thereof decreases when the cell moves over the discharge port after the volume thereof is maximized.
Oil pumps having the above structure are widely used as pumps for lubrication oil in automobiles and as an oil pump for automatic transmissions, etc., since such oil pumps are compact and are simply constructed. When such an oil pump is installed in a vehicle, the oil pump is, for example, driven by the engine of the vehicle in such a manner that the inner rotor of the pump is directly connected to the crankshaft of the engine, which is known as “crankshaft direct drive”.
In such an oil pump, a tip clearance having appropriate size is formed between the tooth tip of the inner rotor and the tooth tip of the outer rotor when the inner and outer rotors are in a phase rotated by 180 degrees from a phase in which the inner and outer rotors engage each other in order to reduce pump noise and to increase mechanical efficiency.
As examples of methods for forming a tip clearance, the profiles of the teeth of the outer rotor may be uniformly cut so as to form clearance between the surfaces of the teeth of the inner and outer rotors and so as to form a tip clearance between the tips of the teeth of the inner and outer rotors in an engagement state, or alternatively, the cycloid curve defining the shape of the teeth may be partially flattened.
Next, conditions, which must be satisfied when the profiles of the teeth of the inner and outer rotors are determined, will be explained below.
With regard to the inner rotor ri, because the sum of the rolling distance of a first circumscribed-rolling circle ai (whose diameter is øai) and the rolling distance of a first inscribed-rolling circle bi (whose diameter is øbi) must be closed when each of the rolling circles completes rolling along a base circle, i.e., the length of circumference of a base circle di (whose diameter is ødi) of the inner rotor ri must be equal to the length obtained by multiplying the sum of the rolling distance per revolution of the first circumscribed-rolling circle ai and the rolling distance of the first inscribed-rolling circle bi by an integer (i.e., by the number of teeth of the inner rotor ri),ødi=n·(øai+øbi).
Similarly, with regard to outer rotor ro, the length of circumference of a base circle “do” (whose diameter is ødo) of the outer rotor ro must be equal to the length obtained by multiplying the sum of the rolling distance per revolution of a second circumscribed-rolling circle ao (whose diameter is øao) and the rolling distance of a second inscribed-rolling circle bo (whose diameter is øbo) by an integer (i.e., by the number of teeth of the outer rotor ro),ødo=(n+1)·(øao+øbo).
Here, because the inner rotor ri and the outer rotor ro must engage each other, assuming that an eccentric distance between two rotors is “e”,øai+øbi=øao+øbo=2e.
Based on the above equations,
(n+1)·ødi=n·ødo, which must be satisfied when the profiles of the inner rotor ri and outer rotor ro are determined.
Here, in order to allocate a clearance (=s) to a clearance between a tooth space and a tooth tip in an engagement phase and to another clearance between the tips (a tip clearance) in a phase rotated by 180 degrees from the engagement phase, the first and second circumscribed-rolling circles and the first and second inscribed-rolling circles are formed so as to satisfy the following equations:øao=øai+s/2;andøbo=øbi−s/2.
More specifically, by increasing the diameter of the circumscribed-rolling circle of the outer rotor, as shown in FIG. 8, a clearance of s/2 is formed between the tooth space of the outer rotor ro and the tooth tip of the inner rotor ri in the engagement phase. On the other hand, by decreasing the diameter of the inscribed-rolling circle of the inner rotor, as shown in FIG. 9, a clearance of s/2 is formed between the tooth space of the inner rotor ri and the tooth tip of the outer rotor ro in the engagement phase.
The oil pump rotor assembly formed such that the above equations are satisfied are shown in FIGS. 7 to 9. Dimensions in the oil pump rotor assembly are as follows:                ødi (the diameter of the base circle di of the inner rotor ri)=52.00 mm; øai (the diameter of the first circumscribed-rolling circle ai)=2.50 mm; øbi (the diameter of the first incribed-rolling circle bi)=2.70 mm; the number of teeth Zi=n=10; the outer diameter of the outer rotor ro is 70 mm; ødo (the diameter of the base circle “do” of the outer rotor ro)=57.20 mm; øao (the diameter of the second circumscribed-rolling circle ao)=2.56 mm; øbo (the diameter of the second incribed-rolling circle bo)=2.64 mm; the number of teeth Zo=n+1=11; and the eccentric distance “e”=2.6 mm.        
As shown in FIGS. 8 and 9, between the external teeth of the inner rotor and the internal teeth of the outer rotor, there are provided not only a radial clearance of s1 at the middle points of the tooth tip and the tooth space but also a circumferential clearance of s2 at the vicinity of the intersecting point of the base circles and the tooth surfaces.
If a clearance of “s” is formed by properly selecting the diameter of the second circumscribed-rolling circle ao and the diameter of the second incribed-rolling circle bo while setting the radial clearance s1 to be s/2, the circumferential clearances s2 become large as shown in FIGS. 8 and 9, and as a result, rattle and tooth surface slip between the inner rotor and the outer rotor are increased; therefore, problems are encountered in that loss in transmission torque is increased, heat is generated, and noise is emitted due to continual impacts between the rotors.